In a wireless communication system, communication is effected by a transmitter transmitting a radio frequency (RF) signal, which is to be picked up by a receiver.
Multi-path fading occurs where the signal travels from the transmitter to the receiver over multiple propagation paths resulting in the reception of multiple replicas of the same signal which interfere with one another. FIG. 1 is a (highly schematised) illustration of the effects of multi-path fading. Here, a transmit antenna 2 broadcasts a signal over an area 3 including a number of buildings 6 or other obstacles which obstruct and/or reflect the signal. The result is that, at any given point, the received signal will be a superposition of multiple replicas of the signal received via multiple propagation paths. Over a range of positions, the received signal intensity thus displays a pattern of peaks and troughs caused by constructive and destructive interference of the different propagation paths.
Note that FIG. 1 is not to scale. In fast-fading conditions, the signal intensity can change measurably due to fading effects over a matter of meters or even centimeters depending on the obstacles and the wavelength of the signal. This means that there may be noticeable fluctuations in signal quality as the receivers move about. In the case where the receiver is a user equipment (UE) terminal such as a mobile phone or laptop, the user will experience a noticeable difference in the signal quality. When comparing the signals at the two receive antennas what matters is not only the signal intensity, but the amplitude and phase (and statistical distribution) of the complex tap-weights that constitute the channel response.
Spatial receive diversity is a technique whereby a receiver is provided with a plurality of physically separated antennas, e.g. the antennas 41 and 42 as shown in FIG. 1. Each receive antenna corresponds to a respective propagation channel, which in this context results from the multiple propagation paths as experienced at a given receive antenna from the transmit antenna 2. Note again that FIG. 1 is not to scale: the two receive antennas 41 and 42 are typically housed within the same terminal, for example within the same mobile terminal.
Space diversity reception is a well known means for improving the performance of a wireless communication system, as described e.g. in J. G. Proakis, “Digital Communications”, New York: McGraw-Hill, 1995, P. Balaban and J. Salz, “Optimum Diversity Combining and Equalization in Digital Data Transmission with Application to Cellular Mobile Radio—Part I: Theoretical Considerations”, IEEE Transactions on Communications, vol. 40, no. 5, pp. 885-894, May 1992, and J. H. Winters, J. Salz, and R. D. Gitlin, “The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems”, IEEE Transactions on Communications, vol. 42, no. 2/3/4, pp. 1740-1751, February 1994. The presence of multiple receive antennas provides the receiver with multiple replicas of the desired signal, transmitted over distinct propagation channels. For sufficient spatial separation between the receive antennas (relative to the carrier wavelength of the radio transmission), the received signals at the different antennas are characterized by uncorrelated channels (i.e., have channel impulse responses with independently fading tap-weights). This system realizes a diversity gain, which can be exploited to improve the error performance of the receiver.
The diversity gain and the corresponding performance advantage decrease for an increased correlation between the diversity channels. However, even in the presence of correlated channels, antenna diversity can still provide a performance advantage in terms of a power gain, which is maximum when the signals on the different diversity branches are affected by uncorrelated disturbance (sum of noise and interference). For instance, a dual-antenna receiver with perfectly correlated diversity channels and uncorrelated noise on the two diversity signals provides a 3 dB gain in terms of signal-to-noise power ratio (SNR). Again, this gain decreases for an increased correlation of the disturbance on the different diversity branches.
The use of multiple receive antennas has been considered for wireless cellular systems like 3GPP Wideband Code Division Multiple-Access (WCDMA) and High-Speed Downlink Packet Access (HSDPA). Examples are given in R. Love, K. Stewart, R. Bachu, and A. Ghosh, “MMSE Equalization for UMTS HSDPA”, IEEE Vehicular Technology Conference, vol. 4, Orlando, Fla., October 2003, pp. 2416-2420, and M. J. Heikkila and K. Majonen, “Increasing HSDPA Throughput by Employing Space-Time Equalization”, IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, vol. 4, Barcelona, Spain, September 2004, pp. 2328-2332.
Although space diversity provides an improvement of the error performance of the receiver, the increased “dimensionality” of the receiver also incurs an increased computational cost. Particularly in the case of a mobile terminal receiver for example, it is important to consider that the performance advantage provided by receive diversity comes at the cost of additional complexity and power consumption, deriving not only from the requirement of multiple antenna units and RF chains, but also from the increased dimensionality of the receiver processing functions required to perform signal detection.
It would be advantageous to benefit from the improved performance of receive diversity whilst avoiding some of the computational cost.